1. Field of the Invention
The invention relates to a cell for the molten salt electrolytic production of aluminum which includes a steel shell lined with graphite blocks, a heat retarding insulating layer between the shell and the lining and cathodic current conductors inserted into the lining.
2. Description of the Prior Art
Cells for producing aluminum by electrolysis of aluminum oxide which is dissolved in a fluoride melt, consist of a trough-shaped cathode part which receives the melted electrolyte and the cathodically deposited fused aluminum. Metallic materials are resistant only to a limited degree against the electrolyte and the electrolysis products at an electrolyte temperature of 940.degree. to 980.degree. C. and must therefore be protected against the attack of the electrolyte and electrolysis products. The cathodic part of the electrolysis cell customarily consists of a trough or a shell of steel referred to as a shell which is lined with a material which is resistant to temperature and corrosion under conditions of fusion-electrolysis of aluminum. The lining also connects the actual cathode which consists of fused aluminum, to the cathodic current conductors or bus bars, which means that the material must also be a good electric conductor. Therefore, carbon and graphite blocks are used almost exclusively for lining the shell. The blocks are connected to each other by carbon-containing tamping and cementing compounds and form a layer which is impervious to the fused metal and electrolyte.
The operability of the lining is determined essentially by its chemical and thermal stability and its electric resistance. In the operation of the electrolysis cell, joule heat is developed in the lining which in part is necessary for adjusting the electrolysis temperature. Because of the temperature difference between the electrolyte and the shell, major power losses through heat conduction can be avoided only if the thermal resistance of the lining is very high. To reduce the losses, a heat insulating layer of ceramic insulating material is customarily arranged between the lining of carbon or graphite blocks and the shell. Although the lining and the heat insulating layer are a functional unit, it has heretofore not been recognized that the lining and the heat-retarding insulating layer form a unit advantageous for the electrolysis operation if the material properties and the geometric design are matched to each other. Replacing carbon blocks by graphite blocks without simultaneous change of the heat insulation has no major effect for this reason, although graphite has a comparatively lower electric resistance and is more resistant to electrolytes than carbon. Thus, it is known, for instance, from U.S. Pat. No. 3,369,986 to line the shell alternatively with carbon blocks and graphite blocks without change of the heat insulation, although the electric resistance of the linings relate to each other approximately 4:1 and the measured voltage drop in the linings approximately 2.5:1. According to German Pat. No. 21 05 247 (British equivalent GB-PS 1 362 933), the variance of the cathodic current density is improved by a lining which contains carbon blocks and graphite blocks both. Instead of the graphite blocks, carbon-bonded graphite blocks are also used (semi-graphite, hard-graphite), without the geometry and type of heat insulation being matched to the changed material properties. It is also known that blocks which consists substantially of petroleum coke and are heated to a high temperature, preferably at least 2000.degree. C. are especially resistant to the electrolyte (German Published Non-Prosecuted Application DE-OS 21 12 287 U.S. equivalent U.S. Pat. No. 4,046,650). The properties of these blocks are approximately: Bulk density -1.57 g/cm.sup.3, porosity -27%, electric resistivity -14 .mu..OMEGA.m. Nothing has become known regarding the nature of the heat retarding layer.
The heat retarding layer consists customarily of refractory blocks or powders of a thickness of between 50 and 250 mm (U.S. Pat. No. 3,434,957) and it is also known that the heat retarding layer constitutes several individual layers (U.S. Pat. No. 3,723,286). Finally, it is known to change the temperature gradients between the bottom and the lateral part of the lining by special insulating elements between these parts (U.S. Pat. No. 4,118,304). These measures are not matched to the material quality of the lining and their effects are accordingly limited.